Semiconductor device and method of fabricating the same

ABSTRACT

After a copper diffusion preventing film  4  is formed on a copper pad  1,  a barrier metal including a titanium film  5,  a nickel film  6,  and a palladium film  7  is formed on the copper diffusion preventing film  4.  The copper diffusion preventing film formed on the copper pad suppresses diffusion of copper. Even when a solder bump is formed on the copper pad, diffusion of tin in the solder and copper is suppressed. This prevents formation of an intermetallic compound between copper and tin, so no interface de-adhesion or delamination occurs and a highly reliable connection is obtained. This structure can be realized by a simple fabrication process unlike a method of forming a thick barrier metal by electroplating. In this invention, high shear strength can be ensured by connecting a solder bump, gold wire, or gold bump to a copper pad without increasing the number of fabrication steps.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor device and, moreparticularly, to a device having a copper pad and a method offabricating the same.

[0002] Recently, in the field of semiconductor devices, wiring layers ina semiconductor chip are formed by using copper, instead of aluminum,for reasons of preventing signal delay and the like. In this case, padsformed on the surface of a semiconductor chip are also formed usingcopper like copper wiring.

[0003] The following three methods are used to mount a semiconductorchip on a wiring substrate and electrically connect them.

[0004] The first method is generally called flip chip mounting by whicha semiconductor chip is mounted in a vertically inverted state on awiring substrate. Solder bumps are formed on copper pads of thesemiconductor chip. The semiconductor chip is mounted on the wiringsubstrate via the solder bumps, and a layer of an encapsulating resin isformed between them.

[0005] Solder balls arranged in the form of an array are formed on theopposite side of the wiring substrate and connected to a printed circuitboard (not shown) or the like.

[0006] A method relevant to the present invention by which a solder bumpis formed on a copper pad will be described below.

[0007]FIGS. 18A to 18E show a semiconductor device fabrication method offorming a copper pad in order of steps.

[0008] As shown in FIG. 18A, a copper pad 101 is formed on the surfaceof a silicon substrate 103. With the surface of this copper pad 101exposed, the silicon substrate 103 is covered with a passivation film102.

[0009] As shown in FIG. 18B, a titanium film 104, a nickel film 105, anda palladium film 106 are stacked in this order on the entire wafersurface by sputtering or evaporation, thereby forming a barrier metal.

[0010] As shown in FIG. 18C, this barrier metal is coated with a resist,and a hole is formed to obtain a resist film 107. In this hole, solderplating is formed as a low-melting metal film 108 for forming aprojecting electrode.

[0011] As shown in FIG. 18D, the resist film 107 is removed, and thePd/Ni/Ti films 104, 105, and 106 forming the barrier metal are etched.

[0012] The whole semiconductor wafer is coated with a flux and heated ina nitrogen atmosphere to reflow the solder.

[0013] The second method is wire bonding. As shown in FIG. 19, a copperpad 302 is formed on a silicon substrate 300. With the surface of thiscopper pad 302 exposed, the silicon substrate 300 is covered with apassivation film 301. A gold wire 304 is connected to the copper pad 302of this semiconductor chip. After this bonding connection, thesemiconductor chip is mounted on a wiring substrate and encapsulatedwith a molding resin.

[0014] The third method uses TAB (Tape Automated Bonding). That is, agold bump is formed on a pad of a semiconductor chip. The semiconductorchip is mounted on a metal cap, and a polyimide tape on which wiring isformed is connected to the gold bump.

[0015] Unfortunately, the aforementioned semiconductor devices have thefollowing problems. As described above, a copper pad of a semiconductorchip is subjected to (1) flip chip mounting using a solder bump, (2)connection by bonding to a gold wire, or (3) TAB mounting using a goldbump. The problems of these methods will be separately described below.

[0016] (1) To form a solder bump on a copper pad, a metal stacked filmis formed to suppress diffusion of tin in the solder. However, copper inthe copper pad reaches the solder through this metal stacked film andforms an intermetallic compound of tin and copper. As a consequence, theshear strength lowers when the device is left to stand at hightemperatures.

[0017] (2) In bonding connection of a copper pad and a gold wire, it isdifficult to connect gold and copper by ultrasonic waves commonly used.

[0018] (3) To form a gold bump on a copper pad, a metal stacked film isformed to suppress diffusion of gold. However, copper in the copper padreaches the gold through this metal stacked film. Consequently, theshear strength lowers by diffusion of the gold and copper.

[0019] To avoid the problem of item (1) above, a thick barrier metal ofcopper or nickel can be formed on a copper pad by electroplating. Inthis method, however, the number of fabrication steps increases by theplating step.

SUMMARY OF THE INVENTION

[0020] The present invention has been made in consideration of the abovesituation, and has as its object to provide a semiconductor devicecapable of ensuring high shear strength by connecting a solder bump,gold wire, or gold bump to a copper pad without increasing the number offabrication steps, and a method of fabricating the same.

[0021] The present invention is a semiconductor device in which asemiconductor element having a copper pad is mounted on a wiringsubstrate, comprising a copper diffusion preventing film formed on thesurface of the copper pad to prevent diffusion of copper, and a metalbump electrically connected to the copper pad with the copper diffusionpreventing film interposed between them, wherein the semiconductorelement is mounted on the wiring substrate via the metal bump.

[0022] The copper diffusion preventing film can contain at least one ofNi, Cr, TiN, TaN, Ta, Nb, and WN.

[0023] The present invention is a semiconductor device in which asemiconductor element having a copper pad is mounted on a wiringsubstrate, comprising a copper diffusion preventing film formed on thesurface of the copper pad to prevent diffusion of copper, a metal filmformed on the surface of the copper diffusion preventing film to improveadhesion between the copper diffusion preventing film and a metal wire,and the metal wire electrically connected to the copper pad with thecopper diffusion preventing film and the metal film interposed betweenthem, wherein the semiconductor element is mounted on the wiringsubstrate via the metal wire.

[0024] The copper diffusion preventing film can contain at least one ofNi, Cr, TiN, TaN, Ta, Nb, and WN, and the metal film can contain one ofAu and Pd.

[0025] The metal bump can contain gold, and one of a stacked film of Ti,Ni, and Pd, a stacked film of Ti, Ni, and Au, a stacked film of TiW andAu, and a stacked film of TiW and Pd can be formed between the copperdiffusion preventing film and the metal bump.

[0026] The metal bump can contain solder, and one of a stacked film ofTi and Ni, a stacked film of Ti, Ni, and Pd, a stacked film of Ti, Ni,and Au, a stacked film of Cr and Ni, a stacked film of Cr and Au, astacked film of Cr, Ni, and Au, a stacked film of Cr, Ni, and Pd, astacked film of Ti and Cu, a stacked film of Ti, Cu, and Au, a stackedfilm of Cr and Cu, and a stacked film of Cr, Cu, and Au can be formedbetween the copper diffusion preventing film and the metal bump.

[0027] The present invention is a method of fabricating a semiconductordevice in which a semiconductor element having a copper pad is mountedon a wiring substrate, comprising the steps of forming a copperdiffusion preventing film for preventing diffusion of copper on thesurface of the copper pad, forming a metal bump to be electricallyconnected to the copper pad with the copper diffusion preventing filminterposed between them, and mounting the semiconductor element on thewiring substrate via the metal bump.

[0028] The present invention is a semiconductor device fabricationmethod of mounting a semiconductor element having a copper pad on awiring substrate by flip chip mounting by using a solder bump,comprising the steps of forming a copper diffusion preventing film forpreventing diffusion of copper on the surface of the copper pad, forminga metal stacked film for suppressing diffusion of tin contained in asolder bump on the copper diffusion preventing film, forming the solderbump on the metal stacked film, and mounting the semiconductor elementon the wiring substrate via the solder bump.

[0029] The metal stacked film can be one of a stacked film of Ti and Ni,a stacked film of Ti, Ni, and Pd, a stacked film of Ti, Ni, and Au, astacked film of Cr and Ni, a stacked film of Cr and Au, a stacked filmof Cr, Ni, and Au, a stacked film of Cr, Ni, and Pd, a stacked film ofTi and Cu, a stacked film of Ti, Cu, and Au, a stacked film of Cr andCu, and a stacked film of Cr, Cu, and Au.

[0030] The present invention is a method of fabricating a semiconductordevice in which a semiconductor element having a copper pad is mountedon a wiring substrate, comprising the steps of forming a copperdiffusion preventing film for preventing diffusion of copper on thesurface of the copper pad, forming a metal film for improving adhesionto a metal wire on the copper diffusion preventing film, electricallyconnecting the metal wire to the copper pad with the copper diffusionpreventing film and the metal film interposed between them, and mountingthe semiconductor element on the wiring substrate via the metal wire.

[0031] The present invention is a method of fabricating a semiconductordevice in which a semiconductor element having a copper pad is mountedon a wiring substrate, comprising the steps of forming a copperdiffusion preventing film for preventing diffusion of copper on thesurface of the copper pad, forming a metal stacked film for preventingdiffusion of gold on the copper diffusion preventing film, forming agold bump to be electrically connected to the copper pad with the copperdiffusion preventing film and the metal stacked film interposed betweenthem, and mounting the semiconductor element on the wiring substrate viathe gold bump.

[0032] In the semiconductor devices and their fabrication methodsaccording to the present invention, a copper diffusion preventing filmformed on a copper pad suppresses diffusion of copper, and thissuppresses diffusion of a component in a metal bump or metal wire andcopper. Therefore, no intermetallic compound of the component in themetal bump or metal wire and copper is formed, so no interfacialde-adhesion removal takes place. Accordingly, a highly reliableconnection is obtained. Additionally, the fabrication process can besimplified compared to a method of forming a thick barrier metal byelectroplating.

[0033] Also, the present invention is a semiconductor device in which asemiconductor element having a copper pad is mounted on a wiringsubstrate, comprising a copper diffusion preventing film formed on thesurface of the copper pad to prevent diffusion of copper, an aluminumfilm formed on the surface of the copper diffusion preventing film, anda metal wire electrically connected to the copper pad with the copperdiffusion preventing film and the aluminum film interposed between them,wherein the semiconductor element is mounted on the wiring substrate viathe metal wire.

[0034] The device can further comprise a metal film for improvingadhesion between the copper diffusion preventing film and the aluminumfilm.

[0035] The copper diffusion preventing film can contain at least one ofNi, Cr, TiN, TaN, Ta, Nb, and WN.

[0036] When the device further comprises the metal film, this metal filmcan contain at least one of Ti, Ni, Cr, TiN, TaN, Ta, Nb, and WN.

[0037] The device can further comprise a passivation film covering thecopper pad or a passivation film covering the copper pad and the copperdiffusion preventing film.

[0038] A method of fabricating this semiconductor device comprises thesteps of forming a copper diffusion preventing film for preventingdiffusion of copper on the surface of the copper pad, forming analuminum film on the copper diffusion preventing film, electricallyconnecting a metal wire to the copper pad with the copper diffusionpreventing film and the aluminum film interposed between them, andmounting the semiconductor element on the wiring substrate via the metalwire.

[0039] The method can further comprise the step of forming a metal filmfor improving adhesion between the copper diffusion preventing film andthe aluminum film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIGS. 1A to 1F are longitudinal sectional views showing thestructure of a semiconductor device according to the first embodiment ofthe present invention and a method of fabricating the same in order ofsteps;

[0041]FIGS. 2A to 2E are longitudinal sectional views showing thestructure of a modification of the semiconductor device according to thefirst embodiment and a method of fabricating the same in order of steps;

[0042]FIGS. 3A to 3D are longitudinal sectional views showing thestructure of a semiconductor device according to the second embodimentof the present invention and a method of fabricating the same in orderof steps;

[0043]FIGS. 4A and 4B are longitudinal sectional views showing thestructure of a semiconductor device according to the third embodiment ofthe present invention and a method of fabricating the same in order ofsteps;

[0044]FIGS. 5A to 5F are longitudinal sectional views showing thestructure of a semiconductor device according to the fourth embodimentof the present invention and a method of fabricating the same in orderof steps;

[0045]FIGS. 6A to 6C are longitudinal sectional views showing thestructure of a semiconductor device according to the fifth embodiment ofthe present invention and a method of fabricating the same in order ofsteps;

[0046]FIGS. 7A to 7D are longitudinal sectional views showing thestructure of a semiconductor device according to the sixth embodiment ofthe present invention and a method of fabricating the same in order ofsteps;

[0047]FIGS. 8A to 8E are longitudinal sectional views showing thestructure of a semiconductor device according to the seventh embodimentof the present invention and a method of fabricating the same in orderof steps;

[0048]FIGS. 9A to 9E are longitudinal sectional views showing thestructure of a semiconductor device according to the eighth embodimentof the present invention and a method of fabricating the same in orderof steps;

[0049]FIGS. 10A to 10D are longitudinal sectional views showing thestructure of a modification of the semiconductor device according to theeighth embodiment and a method of fabricating the same in order ofsteps;

[0050]FIG. 11 is a graph comparing the shear strength of a solder bumpconnected to a copper pad of the semiconductor devices according to theabove embodiments with that of a semiconductor device relevant to thepresent invention;

[0051]FIG. 12 is a longitudinal sectional view showing a structure whenthe semiconductor device of any of the above embodiments isflip-chip-mounted;

[0052]FIG. 13 is a longitudinal sectional view showing a structure whenthe semiconductor device to which gold wires are connected according toany of the above embodiments is mounted on a wiring substrate andencapsulated with a resin;

[0053]FIG. 14 is a longitudinal sectional view showing a structure whenTAB mounting is performed for a semiconductor chip having copper pads;

[0054]FIGS. 15A to 15E are longitudinal sectional views showing thestructure of a semiconductor device according to the ninth embodiment ofthe present invention and a method of fabricating the same in order ofsteps;

[0055]FIG. 16 is a longitudinal sectional view showing one modificationof the ninth embodiment;

[0056]FIG. 17 is a longitudinal sectional view showing anothermodification of the ninth embodiment;

[0057]FIGS. 18A to 18E are longitudinal sectional views showing thestructure of a semiconductor device relevant to the present inventionand a method of fabricating the same in order of steps; and

[0058]FIG. 19 is a longitudinal sectional view showing a structure whena gold wire is connected by bonding to a semiconductor chip having acopper pad.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] One embodiment of the present invention will be described belowwith reference to the accompanying drawings.

[0060] The structure of a semiconductor device according to the firstembodiment of the present invention and a method of fabricating the samewill be described with reference to FIGS. 1A to 1F.

[0061] A semiconductor chip wafer (6 inches in diameter and 625 μm inthickness) on which a copper pad 1 is formed is prepared. The size ofthis copper pad 1 is 100 μm square. A plurality of such pads aretwo-dimensionally formed at a pitch of 350 μm on the entire surface of asemiconductor chip (10 mm×10 mm). The longitudinal sectional structurein this state is as shown in FIG. 1A. The copper pad 1 is formed on thesurface of a silicon substrate 3. With the surface of this copper pad 1exposed, the surface of the silicon substrate 3 is covered with apassivation film 2 having a thickness of, e.g., 1 μm.

[0062] As shown in FIG. 1B, a copper diffusion preventing film 4 isformed on the entire wafer surface by, e.g., sputtering or evaporation.This copper diffusion preventing film 4 is formed to have a thicknessof, e.g., 1 μm by using Ni, Cr, TiN, TaN, Ta, Nb, or WN.

[0063] As shown in FIG. 1C, a 1,000-Å thick titanium film 5, a 3,000-Åthick nickel film 6, and a 500-Å thick palladium film 7 are formed inthis order on the entire wafer surface by using, e.g., a sputteringsystem or an electron beam evaporation system, thereby forming a barriermetal.

[0064] As shown in FIG. 1D, this barrier metal is coated with a resistfilm 8 having a thickness of about 50 μm. A square hole of 100 μm sideis formed in a portion of this resist film 8, which corresponds to aprospective projecting electrode portion on the copper pad 1. This holeis plated with 50-μm thick solder as a low-melting metal film 9 forforming a projecting electrode.

[0065] When eutectic Sn/Pb solder, for example, is used in this solderplating, the semiconductor wafer having a resist pattern is dipped intoa solution containing 30 g/l of tin, 20 g/l of lead, 100 g/l ofalkanesulfonic acid, and an additive consisting primarily of asurfactant. At a bath temperature of 20° C., plating is performed undermoderate stirring at a current density of 1 A/dm² by using the barriermetal as a cathode and an Sn/Pb plate as an anode.

[0066] As shown in FIG. 1E, the resist film 8 is removed by using asolvent such as acetone or a release agent, and the Pd/Ni/Ti film as thebarrier metal is etched.

[0067] That is, an aqua regia-based etchant is used to etch thepalladium film 7 and the nickel film 6. An ethylenediaminetetraaceticacid-based etchant can be used to etch the titanium film 5.

[0068] Subsequently, the copper diffusion preventing film 4 is alsopatterned by etching.

[0069] Finally, the whole semiconductor wafer is coated with a flux andheated in a nitrogen atmosphere at 220° C. for 30 sec to reflow thelow-melting metal film 9.

[0070] After that, electrical tests are conducted, the wafer is dicedinto semiconductor chips, and flip chip mounting is performed. That is,a semiconductor chip and a mounting substrate are temporarily fixed andpassed through a nitrogen reflow furnace to melt the low-melting metalfilm 9 forming the bump. Consequently, the semiconductor chip is mountedas it is electrically connected to the mounting substrate, therebyobtaining a semiconductor device.

[0071] A connection pad on the wiring substrate is one of Cu, Ni, Au,and Pd or a stacked film or mixed film of these metals. Alternatively,one of low-melting metals such as Sn, pb, Ag, Bi, Zn, In, Sb, Cu, Bi andGe or a mixed film of these metals is formed on a metal film.

[0072] A semiconductor device can also be formed by filling and latercuring a silicone resin between the semiconductor chip, which is mountedon the mounting substrate, and the mounting substrate. An epoxy resin oran acrylic resin can also be used as the resin.

[0073] After the semiconductor device was fabricated in accordance withthe abovementioned stesp, a temperature cycle test was conducted toexamine its reliability.

[0074] A square chip of 10 mm side on which 900 bumps were formed wasused as a semiconductor ship, and this chip was mounted on a resinsubstrate to form a sample. In the temperature cycle test, thetemperature was maintained in −65° C. for 30 min, 25° C. for 5 min, and150° C. for 30 min in one cycle.

[0075] Even after 3,000 such cycles, no breaking was found in portionswhere solder bumps were connected to the copper pads 1.

[0076] Also, as shown in FIG. 11, the shear strength of the metal bumphad no problem against a force of 50 gf even after the sample was leftto stand at a high temperature of 150° C. for 1,000 hr. That is, neitherbump removal nor strength degradation occurred, and no short circuitbetween bumps took place.

[0077] Furthermore, the bump shear strength did not lower even after thesemiconductor wafer was left to stand at 150° C. for 5,000 hr.

[0078] In this sample, Sn/Pb solder was used as the solder bump.However, it was also possible to use a solder bump formed by any ofmetals such as Sn, Pb, Ag, Bi, Zn, In, Sb, Cu, Bi, and Ge or by a mixedfilm of these metals.

[0079] In the above embodiment, a titanium/nickel/palladium film is usedas a barrier metal. However, it is also possible to use any of atitanium/nickel stacked film, titanium/nickel/gold stacked film,chromium/nickel stacked film, chromium/gold stacked film,chromium/nickel/gold stacked film, chromium/nickel/palladium stackedfilm, titanium/copper stacked film, titanium/copper/gold stacked film,chromium/copper stacked film, and chromium/copper/gold stacked film.

[0080] In this embodiment as described above, a copper diffusionpreventing film formed on a copper pad suppresses diffusion of copper,and this prevents diffusion of tin in a solder bump and copper.Therefore, no intermetallic compound of copper and tin is formed, so nointerface removal occurs. Consequently, a highly reliable connection isobtained. Also, the fabrication process can be simplified compared to amethod of forming a thick barrier metal by electroplating.

[0081] In this embodiment, the titanium film 5, the nickel film 6, thepalladium film 7 are formed on the copper diffusion preventing film 4,and the low-melting metal film 9 is formed on top of these films.However, as shown in FIGS. 2A to 2E, a low-melting metal film 9 can alsobe formed directly on a copper diffusion preventing film 4.

[0082] The structure of a semiconductor device according to the secondembodiment of the present invention and a method of fabricating the samewill be described below.

[0083] As shown in FIG. 3A, similar to the steps shown in FIGS. 1A and1B in the first embodiment described above, a copper pad 11, apassivation film 12, and a copper diffusion preventing film 14 areformed on a silicon substrate 13. In addition, a resist film 20 isformed on the entire surface of the silicon substrate 13.

[0084] As shown in FIG. 3B, the resist film 20 developed such that itsportion corresponding to a hole in the passivation film 12 on the copperpad 11, and a resist film 20 portion larger than this hole remains. Thesize of this residual resist film 20 is, e.g., 70 μm square.

[0085] As shown in FIG. 3C, the resist film 20 is used as a mask to etchthe copper diffusion preventing film 14, thereby patterning the film 14such that it remains in a portion corresponding to the hole. After that,as in the above first embodiment, a titanium film 15, a nickel film 16,a palladium film 17, and a low-melting metal film 19 are formed as shownin FIG. 3D.

[0086] In this embodiment, the copper diffusion preventing film 14formed on the copper pad 11 is so patterned as to correspond to the holebefore the barrier metal layer is formed. Therefore, only the barriermetal needs to be etched, so the fabrication process is simple.

[0087] Finally, as in the first embodiment, the semiconductor wafer iscoated with a flux and heated in a nitrogen atmosphere at 220° C. for 30sec to reflow the low-melting metal film 19. After that, electricaltests are conducted, dicing is performed to divide the wafer into chips,and flip chip mounting is performed.

[0088] In this embodiment, effects similar to the aforementioned firstembodiment can be obtained. In the first embodiment, the low-meltingmetal film and the barrier metal are simultaneously etched. Therefore,damage to the low-melting metal film may increase depending on the typeof etchant. In this second embodiment, however, the copper diffusionpreventing film is patterned before the barrier metal is formed on theentire surface. This can reduce etching damage to the low-melting metalfilm.

[0089] The structure of a semiconductor device according to the thirdembodiment of the present invention and a method of fabricating the samewill be described below with reference to FIGS. 4A and 4B.

[0090] As in the first and second embodiments described above, asemiconductor wafer (6 inches in diameter and 625 μm in thickness)having a copper pad 31 is prepared.

[0091] As shown in FIG. 4A, a copper diffusion preventing film 34 isformed on the copper pad 31 by electroless plating. As this copperdiffusion preventing film 34, Ni, Cr, TiN, TaN, Ta, Nb, or WN is used.Electroless plating can selectively form the copper diffusion preventingfilm 34 only on the copper pad 31. When the copper diffusion preventingfilm 34 is to be formed by selecting Ni, Ni-B or Ni-P can also be used.The subsequent steps are similar to the abovementioned first and secondembodiments. Consequently, as shown in FIG. 4B, a titanium film 35, anickel film 36, a palladium film 37, and a low-melting metal film 39 areformed.

[0092] In this third embodiment, the copper diffusion preventing film 34is selectively formed on the copper pad 31 by electroless plating.Hence, the copper diffusion preventing film formed on the copper padneed not be etched by patterning unlike in the first and secondembodiments. This simplifies the fabrication process.

[0093] Finally, the semiconductor wafer is coated with a flux and heatedin a nitrogen atmosphere at 220° C. for 30 sec to reflow the soldermetal. After that, electrical tests are conducted, the wafer is dicedinto semiconductor chips, and flip chip mounting is performed.

[0094] In this embodiment, the low-melting metal film is formed on thecopper pad by electroless plating. This allows easy formation of themetal film. Electroless plating can smoothen roughness on the surface ofthe copper pad to thereby improve the adhesion to the barrier metal, andcan also improve the barrier characteristics.

[0095] The structure of a semiconductor device according to the fourthembodiment of the present invention and a method of fabricating the samewill be described below.

[0096] As shown in FIG. 5A, a semiconductor wafer (6 inches in diameterand 625 μm in thickness) having a copper pad 41 is prepared. In thisstage, no passivation film is formed. The size of the copper pad 41 is100 μm square. A plurality of such copper pads are two-dimensionallyformed at a pitch of 350 μm on the entire surface of a semiconductorchip (10 mm×10 mm).

[0097] As shown in FIG. 5B, a copper diffusion preventing film 44 isformed on the entire surface of the semiconductor wafer. As this copperdiffusion preventing film 44, Ni, Cr, TiN, TaN, Ta, Nb, or WN is used.

[0098] As shown in FIG. 5C, a resist film is formed on the wholesemiconductor wafer surface, and exposure and development are performedsuch that the resist film 40 remains on the copper pad 41 as shown inFIG. 5D.

[0099] In FIG. 5D, the copper diffusion preventing film 44 is patternedby etching so as to remain only on the copper pad 41.

[0100] As shown in FIG. 5E, a passivation film 42 is formed. As thispassivation film 42, an inorganic film such as SiN or SiO₂, an organicfilm such as polyimide, BCB, or epoxy resin, or a composite film ofthese films is used.

[0101] The above steps are similar to the first embodiment. In thisembodiment, the copper diffusion preventing film 44 is formed first onthe copper pad 41, so only a barrier metal needs to be etched.

[0102] Finally, the semiconductor wafer is coated with a flux and heatedin a nitrogen atmosphere at 220° C. for 30 sec to reflow a low-meltingmetal film 49 as shown in FIG. 5F.

[0103] After that, electrical tests are conducted, the wafer is dicedinto semiconductor chips, and flip chip mounting is performed.

[0104] The structure of a semiconductor device according to the fifthembodiment of the present invention and a method of fabricating the samewill be described below with reference to FIGS. 6A to 6C.

[0105] As shown in FIG. 6A, a semiconductor wafer (6 inches in diameterand 625 μm in thickness) having a copper pad 51 is prepared. In thisstage, no passivation film is formed. The size of the copper pad 51 is100 μm square. A plurality of such copper pads are two-dimensionallyformed at a pitch of 350 μm on the entire surface of a semiconductorchip (10 mm×10 mm).

[0106] As shown in FIG. 6B, the copper pad 51 and a copper wiringportion are electroless-plated to selectively form a copper diffusionpreventing film 54. As this copper diffusion preventing film 54, Ni, Cr,TiN, TaN, Ta, Nb, or WN is used. Instead of Ni, Ni-B or Ni-P can also beused. As in the fourth embodiment described above, the copper diffusionpreventing film 54 is selectively formed on the copper pad 51. Hence,the subsequent steps are identical with FIGS. 5E and 5F in the fourthembodiment.

[0107] Finally, the semiconductor wafer is coated with a flux and heatedin a nitrogen atmosphere at 220° C. for 30 sec to reflow a low-meltingmetal film 59, as shown in FIG. 6C.

[0108] After that, electrical tests are conducted, dicing is performedto divide the wafer into chips, and flip chip mounting is performed.

[0109] In this embodiment, the low-melting metal film is formed usingelectroless plating, so the fabrication process can be simplified. Also,the use of this electroless plating can smoothen roughness on thesurface of the copper pad to thereby improve the adhesion to the barriermetal, and can also improve the barrier characteristics.

[0110] The sixth embodiment of the present invention will be describedbelow with reference to FIGS. 7A to 7D.

[0111] In the first to fifth embodiments described above, printing isused to form solder bumps. In this embodiment, however, solder paste isburied in a hole by using a squeegee.

[0112] As shown in FIG. 7A, as in the first embodiment describedpreviously, a copper diffusion preventing film 64 is formed on a copperpad 61. After that, a titanium film 65, a nickel film 66, and apalladium film 67 are formed as a barrier metal, and a resist film 68 isalso formed. This resist film 68 has a square pattern of, e.g., 100 μmside.

[0113] The resist film 68 is used as a mask to pattern the titanium film65, the nickel film 66, and the palladium film 67 by etching. The copperdiffusion preventing film 64 is also patterned.

[0114] As shown in FIG. 7C, a 60-μm thick printing mask 601 which is,e.g., 160 μm square is aligned on the semiconductor wafer. This printingmask 601 has a printing mask hole as shown. Solder paste is buried aslow-melting metal paste 603 in the printing mask hole by moving asqueegee 602 in the direction of an arrow.

[0115] As shown in FIG. 7D, the printing mask 601 is pulled up towardthe upper portion of the drawing, or the semiconductor wafer is pulleddown toward the lower portion, to leave the solder paste behind in thebarrier metal portion.

[0116] After that, the semiconductor wafer is heated in a nitrogenatmosphere at 220° C. for 30 sec to reflow the solder paste, and a fluxis washed away. After that, electrical tests are conducted, the wafer isdiced into chips, and flip chip mounting is performed.

[0117] In this embodiment, an Sn-Pb solder bump is used as low-meltingmetal paste. However, high reliability can also be obtained by using ametal mixture of, e.g., Sn, Pb, Ag, Bi, Zn, In, Sb, Cu, Bi, and Ge. Notethat although a solder bump can be formed by printing or by burying itin a mask hole by using a squeegee, it can also be formed by placing asolder ball or coating the barrier metal with molten solder.

[0118] In the sixth embodiment as described above, a low-melting metalfilm is formed by printing. This can make the fabrication processsimpler than when the film is formed by plating.

[0119] The structure of a semiconductor device according to the seventhembodiment of the present invention and a method of fabricating the samewill be described below with reference to FIGS. 8A to 8E. Thisembodiment uses a mounting method which bonds a metal wire.

[0120] As shown in FIG. 8A, a semiconductor wafer (6 inches in diameterand 625 μm in thickness) having a copper pad 71 is used.

[0121] The size of this copper pad 71 is 100 μm square. A plurality ofsuch copper pads are arranged at a pitch of 200 μm in a peripheralregion of each semiconductor chip (10 mm×10 mm) on the wafer.

[0122] As shown in FIG. 8B, a copper diffusion preventing film 74 isformed on the upper surface of the copper pad 71 by sputtering orevaporation. As this copper diffusion preventing film 74, Ni, Cr, TiN,TaN, Ta, Nb, or WN is used.

[0123] As shown in FIG. 8C, a gold film 75 is formed on the entiresemiconductor wafer surface by using a sputtering system or electronbeam evaporation. A palladium film can also be formed instead of thegold film. Subsequently, the gold film 75 is coated with a resist (notshown), and this resist is exposed and developed to form a resist film100 Am square.

[0124] This resist film is used as a mask to etch the gold film 75 andthe copper diffusion preventing film 74. When the resist film is removedby a release agent, a shape shown in FIG. 8D is obtained. After that,electrical tests are conducted, and the wafer is diced into chips. Eachchip is placed on a mounting substrate and, as shown in FIG. 8E, mountedas it is electrically connected to the mounting substrate by bonding agold-containing metal wire 76 onto the gold film 75.

[0125] A semiconductor device was fabricated in accordance with theabove steps and subjected to a temperature cycle test to examine itsreliability. In this temperature cycle test, one cycle was −65° C. (30min)→25° C. (5 min)→150° C. (30 min). Even after 3,000 such cycles, thetensile strength of the bonding wire did not lower, and no breaking wasfound. Also, no lowering of the ball shear strength in the wireconnected portion was found.

[0126] In the seventh embodiment as described above, a copper diffusionpreventing film formed on a copper pad suppresses diffusion of copper,and this suppresses diffusion of copper between a metal wire and thecopper. Accordingly, the connection strength between the metal wire andthe copper pad and the reliability improve.

[0127] The structure of a semiconductor device according to the eighthembodiment of the present invention and a method of fabricating the samewill be described below. As shown in FIG. 9A, a semiconductor wafer (6inches in diameter and 625 μm thick) having a copper pad 81 is used. Thesize of this copper pad 81 is 50 μm square. A plurality of such copperpads are formed at a pitch of 60 μm on the periphery of eachsemiconductor chip (10 mm×10 mm) on the semiconductor wafer.

[0128] As shown in FIG. 9B, a copper diffusion preventing film 84 isformed on the upper surface of the copper pad 81 by sputtering orevaporation. As this copper diffusion preventing film, Ni, Cr, TiN, TaN,Ta, Nb, or WN is used.

[0129] As shown in FIG. 9C, a titanium film 85, a nickel film 86, and apalladium film 87 are formed on the entire semiconductor wafer surfaceby using, e.g., a sputtering system or electron beam evaporation.

[0130] Instead of forming the titanium/nickel/palladium film as abarrier metal, it is also possible to use any of a titanium/nickel/goldstacked film, titanium/tungsten/gold stacked film, andtitanium/tungsten/palladium stacked film as another stacked film.

[0131] As shown in FIG. 9D, the palladium film 87 is coated with a 20-μmthick resist, and this resist is exposed and developed to form a hole 50μm square, thereby obtaining a resist film 88. This hole is plated witha 16-μm thick gold film 89.

[0132] As shown in FIG. 9E, the resist film 88 is removed by using asolvent such as acetone or a release agent, and the palladium film 87,nickel film 86, and titanium film 85 forming the barrier metal areetched. The etching of the palladium film 87 and the nickel film 86 isdone by using an aqua regia-based etchant.

[0133] The titanium film 85 can be etched using anethylenediaminetetraacetic acid-based etchant. Furthermore, the copperdiffusion preventing film 84 is etched. After that, electrical tests areconducted, the wafer is diced into chips, and each chip is mounted on atape substrate by TAB by using gold bumps.

[0134] A semiconductor device was fabricated in accordance with theabove steps and subjected to a temperature cycle test to examine itsreliability. In this temperature cycle test, one cycle was −65° C. (30min)→25° C. (5 min)→150° C. (30 min). Even after 3,000 such cycles, theconnection strength of the gold bump did not lower, and no breaking wasfound.

[0135] In the eighth embodiment described above, a copper diffusionpreventing film formed on a copper pad suppresses diffusion of copper,and this suppresses diffusion of the copper to a gold bump. Accordingly,no interface removal occurs, so a highly reliable connection isobtained.

[0136] In the above eighth embodiment, the gold bump 89 is formed afterthe titanium film 85, the nickel film 86, and the palladium film 87 areformed on the copper diffusion preventing film 84.

[0137] As shown in FIGS. 10A to 10D, however, a gold bump 89 can also beformed directly on a copper diffusion preventing film 84 without formingthe titanium film 85, the nickel film 86, and the palladium film 87.

[0138] The ninth embodiment of the present invention will be describedbelow with reference to FIGS. 15A to 15E.

[0139] As shown in FIG. 15A, a silicon substrate 603 (a semiconductorchip wafer (6 inches in diameter and 625 μm in thickness) having acopper pad 601 is prepared. The size of this copper pad 601 is, e.g.,100 μm square. A plurality of such copper pads are formed at a pitch of200 μm on the periphery of a semiconductor chip (10 mm×10 mm). Also,with the surface of this copper pad 601 exposed, the surface of thesilicon substrate 603 is covered with a passivation film 602 having athickness of, e.g., 1 μm.

[0140] As shown in FIG. 15B, a copper diffusion preventing film 604 isformed on the entire surface of the silicon substrate 603 by, e.g.,sputtering or evaporation. This copper diffusion preventing film 604 isformed to have a thickness of, e.g., 1 μm by using Ni, Cr, TiN, TaN, Ta,Nb, or WN.

[0141] As shown in FIG. 15C, an aluminum (Al) film 605 is formed on theentire surface of the silicon substrate 603 by using, e.g., a sputteringsystem or an electron beam evaporation system.

[0142] The whole surface is coated with a resist, and this resist isexposed and developed to form a resist film 100 μm square in a portioncorresponding to the copper pad 601. This resist film is used as a maskto etch the copper diffusion preventing film 604 and the aluminum film605 as shown in FIG. 15D, and the resist film is removed to obtain asemiconductor device.

[0143] After that, electrical tests are conducted, and the wafer isdiced into a plurality of chips. As shown in FIG. 15E, a metal wire 606is bonded on the aluminum film 605 and then bonded to an electrode on awiring substrate (not shown), thereby mounting each chip.

[0144] A semiconductor device obtained by the above steps was subjectedto a temperature cycle test to examine its reliability. In thistemperature cycle test, one cycle was −65° C. (30 min)→25° C. (5min)→150° C. (30 min).

[0145] Even after 3,000 such cycles, the tensile strength of wirebonding did not lower, and no breaking was found. Also, the shearstrength of a ball 607 for connecting the metal wire 606 and thealuminum film 605 did not lower.

[0146] In this embodiment, the metal wire 606 is connected directly tothe aluminum film 605. However, as shown in FIG. 16, an adhesion layer705 can also be formed between an aluminum film 706 and a copperdiffusion preventing film 704. As this adhesion layer, it is possible touse Ti, Ni, Cr, TiN, TaN, Ta, Nb, or WN, or a stacked film of thesemetals. In this modification, a copper pad 701, a passivation film 702,the copper diffusion preventing film 704, the aluminum film 706, and ametal wire 707 are formed in the same manner as in the ninth embodiment,so a detailed description thereof will be omitted.

[0147] Also, as shown in FIG. 17, a passivation film 802 can be formedto cover not only a copper pad 801 but a copper diffusion preventingfilm 804.

[0148] Methods of mounting a semiconductor chip on a wiring substrate byusing any of the above embodiments will be described below withreference to the accompanying drawings.

[0149] First, in a method called flip chip mounting, as shown in FIG.12, a semiconductor chip 204 is vertically inverted and placed on awiring substrate 201. Bumps 203 made of, e.g., solder or gold are formedon copper pads of this semiconductor chip 204. The semiconductor chip204 having this structure is placed on the wiring substrate 201 via thebumps 203, and an encapsulating resin 202 is formed between thesemiconductor chip 204 and the wiring substrate 201. Solder balls 205arranged in the form of an array are formed on the opposite surface ofthe wiring substrate 201 and connected to a printed circuit board (notshown).

[0150] In a mounting method using wire bonding, a metal wire 76 isconnected by bonding as shown in FIG. 8E. After that, as shown in FIG.13, a semiconductor chip 402 is placed on a wiring substrate 401 andencapsulated with a molding resin 405. Solder balls 404 are formed inthe form of an array on the surface away from the mounting surface ofthe wiring substrate 401.

[0151] In a TAB mounting method, as shown in FIG. 14, gold bumps 503 areformed on pads of a semiconductor chip 502. This semiconductor chip 502is placed on a metal cap 506. With a resin substrate 501 interposedbetween them, lines are formed and a polyimide tape 507 having solderballs 505 is connected.

[0152] The aforementioned embodiments are merely examples and hence donot restrict the present invention. For example, although a barriermetal is formed by titanium/nickel/palladium films, this barrier metalcan also be formed using another material. Also, in each of the aboveembodiments, a copper diffusion preventing film is formed on a copperpad connected to a copper line, and a barrier metal is formed on thiscopper diffusion preventing film. However, similar effects can beobtained when a copper diffusion preventing film is formed on a silverpad connected to a silver line and a barrier metal is formed on thiscopper diffusion preventing film.

1.-11. (Canceled).
 12. A method of fabricating a semiconductor device inwhich a semiconductor element having a copper pad is mounted on a wiringsubstrate, comprising the steps of: forming a copper diffusionpreventing film for preventing diffusion of copper on the surface ofsaid copper pad; forming a metal bump to be electrically connected tosaid copper pad with said copper diffusion preventing film interposedtherebetween; and mounting said semiconductor element on said wiringsubstrate via said metal bump.
 13. A semiconductor device fabricationmethod of mounting a semiconductor element having a copper pad on awiring substrate by flip chip mounting by using a solder bump,comprising the steps of: forming a copper diffusion preventing film forpreventing diffusion of copper on the surface of said copper pad;forming a metal stacked film for suppressing diffusion of tin containedin a solder bump on said copper diffusion preventing film; forming saidsolder bump on said metal stacked film; and mounting said semiconductorelement on said wiring substrate via said solder bump.
 14. A methodaccording to claim 13, wherein said metal stacked film is one of astacked film of Ti and Ni, a stacked film of Ti, Ni, and Pd, a stackedfilm of Ti, Ni, and Au, a stacked film of Cr and Ni, a stacked film ofCr and Au, a stacked film of Cr, Ni, and Au, a stacked film of Cr, Ni,and Pd, a stacked film of Ti and Cu, a stacked film of Ti, Cu, and Au, astacked film of Cr and Cu, and a stacked film of Cr, Cu, and Au.
 15. Amethod of fabricating a semiconductor device in which a semiconductorelement having a copper pad is mounted on a wiring substrate, comprisingthe steps of: forming a copper diffusion preventing film for preventingdiffusion of copper on the surface of said copper pad; forming a metalfilm for improving adhesion to a metal wire on said copper diffusionpreventing film; electrically connecting said metal wire to said copperpad with said copper diffusion preventing film and said metal filminterposed therebetween; and mounting said semiconductor element on saidwiring substrate via said metal wire.
 16. A method of fabricating asemiconductor device in which a semiconductor element having a copperpad is mounted on a wiring substrate, comprising the steps of: forming acopper diffusion preventing film for preventing diffusion of copper onthe surface of said copper pad; forming a metal stacked film forpreventing diffusion of gold on said copper diffusion preventing film;forming a gold bump to be electrically connected to said copper pad withsaid copper diffusion preventing film and said metal stacked filminterposed therebetween; and mounting said semiconductor element on saidwiring substrate via said gold bump.
 17. A method of fabricating asemiconductor device in which a semiconductor element having a copperpad is mounted on a wiring substrate, comprising the steps of: forming acopper diffusion preventing film for preventing diffusion of copper onthe surface of said copper pad; forming an aluminum film on said copperdiffusion preventing film; electrically connecting a metal wire to saidcopper pad with said copper diffusion preventing film and said aluminumfilm interposed therebetween; and mounting said semiconductor element onsaid wiring substrate via said metal wire.
 18. A method according toclaim 17, further comprising the step of forming a metal film forimproving adhesion between said copper diffusion preventing film andsaid aluminum film.